1. Field of the Invention
The present invention relates to a network filtering circuit, and more particularly to a network filtering circuit for protection lightning surge.
2. Description of Related Arts
U.S. Pat. No. 8,272,898 discloses a network filtering circuit including a cable side, a physical side, and a plurality of transmission channels connected therebetween. Each of the transmission channels includes an isolation transformer with a primary coil connected to the cable side and a second coil connected to a common mode choke wherein the other side of the common mode choke is connected to the physical side. The primary coil has a center tap series connection with a capacitor and a resistor to a ground node. A surge would follow from the cable side to the ground node through the capacitor and the resistor, when the cable side is bearing a lightning. The capacitor and the resistor maybe damaged by the surge, after that the surge maybe coupled to the secondary coil from the primary coil and conducted to the physical side which has a network chip, therefore the network chip may be damaged by the surge.
U.S. Patent Application Publication No. 2014/0160608 discloses a network filtering circuit including a plurality of transmission channels connected between a cable side and a physical side. Each channel includes two parallel transmission routes. Each channel includes an autotransformer in parallel connection with the two routes and two capacitors series connected with the two routes, respectively. The autotransformer has a center tap directly connected to a ground node and simultaneously connected with the cable side and the physical side.
Therefore, a network filtering circuit for protection lightning surge is desired.